Muzzle-loading rifles required a bullet smaller than the bore so it could easily be rammed into the muzzle and then, paradoxically, as large as the bore so that upon firing it would snugly fill the grooves and fully use the force of the powder gases. A number of inventors had already devised self-expanding bullets when, in 1836, Greener developed such a bullet consisting of a flat-ended oval ball with a cavity in which a metal plug was inserted. When the gun fired, the plug drove forward and caused the bullet to expand and engage the rifle grooves. Greener submitted his invention to the British government, but it was rejected; later, when a French captain, Claude Minié, received £20,000 from the British government for a similar bullet, Greener sued for plagiarism and ultimately was awarded £1,000. Greener was the author of several books on gunnery; he also patented inventions in other fields, such as a process (co-invented by W.E. Strait) for the manufacture of pencils from the hard graphite carbon deposited in the interior of gas retorts.

His son, William Wellington Greener (1834–1921), invented a new type of shotgun mechanism and wrote several books, including The Gun and Its Development (1881).